Pathogenic bacteria overcome host defenses to establish infections at mucosal surfaces and in mammalian tissues. Thus, understanding bacterial-host interactions that occur at these sites is critical to understanding host defenses and immunity. Pathogenic Yersinia spps, Y. pseudotuberculosis, Y. pestis and Y. enterocolitica all contain a virulence plasmid, pYV, that encodes components of a type III secretion apparatus as well as effector proteins, called Yops. The type III secretion system injects Yops into mammalian cells where they disrupt and/or alter mammalian cell function. Most Yops have multiple protein targets and biochemical activities in cultured cells. However, their cellular targets in an animal infection model are unknown. YopH and YopE are crucial virulence factors that allow Yersinia to colonize many tissues and cause disease. By studying mutants of YopH and YopE that are defective in acting on specific host proteins in an animal model system of infection, the protein targets that YopH and YopE must inactivate to enable Yersinia to colonize tissues and cause disease will be defined. In addition, the host cells that are injected with YopH and YopE during infection and the host cell types that kill yopH and yopE mutants in different tissues will be identified. Combined, this knowledge will reveal the host defenses targeted by YopH and YopE in different tissues and the specific activities of YopH and YopE needed to thwart these host defenses to enable Yersinia to colonize and cause disease. The long-term goals of this project are to understand the role of all Yops for Yersinia spp. to colonize and cause disease in animal tissues and to understand the host defenses that prevent, combat and/or contain Yersinia infection in tissues.